Under Pressure

Air compressors are indispensable in today’s collision repair shops. Here’s a look at different styles of air compressors, and necessary maintenance to keep them running at peak efficiency.

The air compressor is a cornerstone piece of equipment in collision repair shops. Without one, no repairs can be done. While it is obvious that compressed air is needed for your paint guns, you also need compressed air for your pneumatic tools that are used to remove damaged fenders, doors and hoods.

Let’s look at the indispensable air compressor—how it works and what you need to do to keep it working at peak efficiency.

Air Compressor Basics

There are three basic types of air compressors: (1) single stage, (2) two-stage and (3) rotary screw. The single stage compressor develops compressed air as the result of the movement of a reciprocating piston. Atmospheric air pressure is 14.7 pounds per square inch (psi). A compressor of this type can achieve a maximum pressure of 125 psi.

A pressure rating of 125 psi might not be sufficient for your body shop. Many body shops utilize a two-stage compressor, which consists of two cylinders of different bore sizes. In the first, and larger cylinder, atmospheric air is compressed to around 40 psi. The second stage, the compressed air from the large cylinder passes through an intercooler to reduce the temperature (remember, the temperature of the air increases as it is compressed). This air then transfers to the second, and smaller, cylinder where it is compressed to about 175 psi, which is sufficient for all the tasks performed in today’s body shops.

The rotary screw compressor compresses air by forcing it between two spinning rotors. As the rotors turn, the air is forced into a smaller volume, thus raising the pressure. In the same manner as a rotary internal combustion engine, the rotary screw compressor has a lot fewer parts than its piston counterpart, is much quieter and produces a steady stream of compressed air without the pulses produced by piston compressors as the intake and exhaust valves open and close. The rotary screw compressor can easily produce 175 psi. Used as standard equipment in many industries, this type of compressor has never been embraced by the automotive refinishing business because of concerns over oil output problems.

As the air is compressed, it is diverted to an air tank, where it is held until needed. Working air pressure is not available until the tank pressure exceeds the pressure rating of the tool you are using. As a rule of thumb, the larger the air tank the longer you can work at the required pressure before the tank has to be refilled.

Air Compressor Ratings

In order to compare one air compressor to another, specifications are listed that indicate performance. The most common specification that you probably see when you first check out an air compressor is horsepower. As it is with an automobile, horsepower is a rating of the amount of work performed. On an air compressor, the horsepower rating applies to the motor that drives the compressor. In body shops, you’ll see air compressors with ratings ranging from 3 horsepower to 25 horsepower.

You can assume that the higher the horsepower rating of a compressor, the more powerful it is. And, generally speaking, the other specifications by which a compressor is measured will increase as horsepower increases.

Cubic feet per minute (CFM) is another common performance rating for air compressors. CFM is the volume of air being delivered to the air tool. This rating is a measure of the capability of the compressor. Obviously, a compressor with a higher CFM rating will deliver more air than one with a lower CFM rating.

Let’s take a closer look at this CFM rating. Displacement CFM is the theoretical volume of air the compressor can generate in one minute. The free air CFM is the actual volume of air that is pumped in one minute at working pressure. Another way to look at this rating is to think of it as the net CFM rating. The volumetric efficiency of the compressor is simply the free air CFM divided by the displacement CFM. Or, if you know the volumetric efficiency, you can compute the free air CFM by multiplying the displacement CFM by the volumetric efficiency. Free air CFM should be the primary CFM rating that you should check when buying a new air compressor.

Finally, the last performance rating is pressure (psi). It is the measure of the air pressure delivered to the air tool by the compressor. It is usually listed as the continuous air pressure or the maximum air pressure.

Air Compressor Maintenance

As, arguably, the most important piece of equipment in your body shop (after all, without it, you couldn’t really do much of anything), performing maintenance on the air compressor should be a top priority. Unfortunately, in many shops, this is not the case. The net result, in extreme situations, can be paint jobs that must be redone (due to excessive water and/or dirt in the distribution lines) and rusted pneumatic tools (due to the same water that ruined the paint jobs). As with any major piece of equipment, ongoing maintenance will go a long way to ensure proper operation of your air compressor.

Maintenance should be set up as a scheduled event and categorized as Daily, Weekly and Monthly.

But before we get into specific tasks, let’s discuss water. There are several methods that can be used to drain water from the air compressor system. While it can be done manually, this is not recommended as there are several automatic systems that work much better. Regardless of the system, draining the water should be the top priority.

Timed drains work well, but these systems drain even when no work is being done. Float drains work well, too, but the biggest problem with this system is the build-up of scale, rust and gunk, which can clog the system. The best system is probably an on-demand system that does just that—drains the water when the water needs to be drained, seldom or often, depending on the needs of your shop.

In addition to draining the moisture from the system, other daily maintenance tasks include checking the level of oil in the crankcase. Do not overfill as this will cause excess usage. Speaking of oil, a multi-grade oil can be substituted for a straight weight oil, if necessary. Just remember that such oils contain additives that can cause carbon deposits and varnish.

Weekly maintenance tasks include the following: (1) checking the safety valve (unseat it to be sure it releases air), (2) cleaning the air filters (dirty filters reduce the efficiency of the compressor and increase oil usage), (3) cleaning the compressor fins, body, head and aftercooler (a clean compressor runs cooler) and (4) check the inline oil filter (change the filter, if necessary).

On a monthly basis, these tasks should be performed: (1) change the compressor crankcase oil, (2) adjust the switch cut-in and cut-out settings, as required, (3) check the relief valve, (4) check the belt tensions to prevent slippage; be careful not to overtighten the belts as this can overload the motor, (5) check and correct any air leaks and (6) check and correct any oil leaks.